Conventionally, some diesel engines have a selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flow, said catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen. From a reducing agent tank, a required amount of reducing agent is added upstream of the selective reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby decrease a concentration of NOx discharged.
Effectiveness of ammonia (NH3) used as a reducing agent for reduction and removal of NOx is well known in a field of industrial flue gas denitration in a plant or the like. However, in a field of an automobile where safety is hard to assure as to running with ammonia itself being loaded, recently used as the reducing agent is nontoxic urea water.
For example, Patent Literature 1 is already present as prior document related to the invention.